1. Technical Field
The invention relates to motion sensors and more particularly to a motion sensor with a built in alignment aid.
2. Description of the Problem
A typical passive infrared (“PIR”) motion sensor uses a multiple Fresnel lens system to create a fixed number of detection zones. The optical alignment of each lens of the lens system with the internal infrared detector defines a detection zone that extends outward in front of the sensor. Each detection zone is only a few inches wide near the sensor, but expands at greater distances in a manner determined by the focal length of each lens. Even so, with the typical focal lengths used in PIR motion sensors, the detection zone will only be a few feet wide at a range of fifty feet. In order to achieve adequate sensitivity, the lenses cannot be made arbitrarily small, so a typical motion sensor lens will have about 20 elements in the lens system. If the motion sensor is designed to cover a large area, the relatively small number of detection zones means there will be large portions of the monitored area in which motion cannot be detected. There is no clear indication to the user that indicates where the monitored and un-monitored areas will be. However, to operate properly, the motion sensor must be mounted and aimed so that the detection zones adequately cover the target area. Both the horizontal and vertical mounting angles of the motion sensor must be set properly in order to keep the detection zones within the area that is to be monitored. Even a small error can result in a motion sensing system that does not adequately monitor the target area.
Since the detection zones of a PIR motion sensor are not visible, proper alignment can become quite tedious. During installation the user must essentially guess at the correct sensor angles and then walk around in front of the motion sensor to try to confirm that the detection zones are positioned properly. The motion sensor typically provides an LED or a special test mode to facilitate this walk test. When the user moves through one of the detection zones, either the LED will flash or a light will turn on briefly to indicate that motion has been detected. Due to the nature of the electronics used with motion sensors, the user must then wait a few seconds for the motion sensor to re-stabilize before he can continue the test. Using this trial and error approach, the user can eventually determine the position of each of the detection zones and adjust the motion sensor until the detection zones are positioned properly. Since this process is prone to error and, if done properly, very time consuming, the results of the installation are often less than ideal. A typical problem with PIR motion sensors is that care must be taken to insure that none of the detection zones contains a heat source or other object that might cause false triggers. While such objects are usually listed in the operation manual and are easy to identify, actually determining whether or not such an object is in one of the detection zones can be quite difficult.
In a similar manner, active ultrasonic and microwave motion sensors can be difficult to aim. These types of motion sensors typically have one continuous detection zone rather than a multitude of detection zones, but they also do not provide any visible feedback that allows the user to determine the shape and placement of the detection zone. These types of sensors send a signal into the detection zone (either microwave or ultrasonic) and then measure the reflected signals in order to detect motion. The shape of the detection zone can be controlled by the type of transducers used and their mechanical arrangement on the motion sensor. As with PIR motion sensors, the only way to properly align the motion sensor is to perform the slow and tedious walk around test.
U.S. Pat. No. 6,531,966 describes a device that incorporates a laser pointer with a motion sensor. A visible light pattern is generated by the laser, but the laser pointer is not visible in the detection zones of the motion sensor. Rather, the laser pointer is independently adjustable with respect to the motion sensor. The intent is to use the motion sensor to detect a car entering a parking area. When motion is detected, the motion sensor triggers operation of the laser. The laser pointer is aimed to illuminate a particular spot on the car when it is parked in the proper position. The motion sensor's primary purpose is to conserve battery power by turning off the laser when no motion is detected.
U.S. Pat. No. 6,215,398 describes a device which uses two LED's similar to the test LED used as alignment aids in many PIR motion sensors. The LED's are placed behind the lens and located so that they illuminate the lens from behind whenever motion is detected. They are positioned behind selected lens segments so the segment detecting an observer will look brighter to the observer since it will be better focused where the observer is standing. This approach has several drawbacks. For one, ideally the LED and the PIR detector should be in the same position relative to the lens segment. Since this is not physically possible, LED position is compromised. Also, this technique only works if the lens is relatively clear. It is often desirable to use a lens that has pigments added to make it match a desired color. These pigments block visible light from the LED while allowing infrared energy to pass through. Even without pigments, the material used to make this type of lens is often quite milky and diffuses visible light. When lit from behind, a lens made from this material would diffuse the LED light throughout the lens and defeat the intent of creating a relatively brighter spot if the user were standing in a position that should appear to be more focused. In addition, the lens has only a few, very large lenses and only two LEDS. It would not be practical to extend this approach to a lens system that had a substantially greater number of lens elements. Properly positioning 20 or more LED's behind the corresponding lenses would not allow the differentiation in lens brightness that would be required to identify the correct lens when standing at a distance from the motion sensor. Finally, as with the typical walk test LED, a stop and go approach must be used since the user must stop moving and wait for the motion detecting circuits to stabilize and turn the LED back off each time motion is detected.